This invention relates to an anodically protected heat exchanger, and more particularly to a heat exchanger which has a corrosive liquid on the shell side and is provided with an improved anodic protection system that assures control of voltage within the passive range along the exterior surfaces of the tubes.
Electrical currents are used conventionally in industry to protect metals against corrosion, either by the long known method of cathodic protection or, more recently, by the creation of a protective anodic film, in which case the technique is known as anodic protection. This invention is concerned with improved anodic protection for shell and tube heat exchangers and more particularly with protection of the exterior surfaces of the tubes in exchangers wherein a corrosive liquid is passed through the shell side and the heat transfer fluid passes through the tubes.
In conventional anodic protection systems for such heat exchangers, cathodes are typically positioned at various locations within the heat exchanger shell, or in the inlet and exist nozzles for the corrosive liquid, and all of these cathodes are electrically connected to the negative terminal of a power source in a single anodic protection circuit. Each cathode is of limited dimension and effectively provides protection in a local zone surrounding that cathode. To provide protection for a large shell and tube heat exchanger, these so-called pin cathodes must be positioned at a substantial number of points along the length of the exchanger in order to provide protection of the tubes throughout the exchanger.
Where there is a substantial longitudinal temperature gradient along the shell, the potential at the cathode required to maintain the tube surfaces in the passive potential range may vary significantly from one end of the exchanger to the other. Where the shell side fluid is a highly corrosive liquid, such as sulfuric acid, it may be difficult to control the cathode potential at a level which assures control of the tube surface potential within the passive range along the entire length of the exchanger.
Provision of pin cathodes in numerus locations along the shell is also expensive and multiplies the points at which corrosive liquid may potentially leak from the system. The disadvantages of multiple pin electrodes can be minimized by use of elongate cathodes that extend parallel to the tubes from one tube sheet to the other. However, the voltage drop along such a lengthy cathode may cause the tube surfaces at one end of the exchanger to stray outside the passive range in order to control tubes at the other end within that range. For an aggressive acid, such as sulfuric acid, the passive potential range is generally narrower at high temperature than it is at low temperature, requiring especially careful control of the potential in the hot end of the heat exchanger. As a consequence, the potential may readily stray into the transpassive range at the cold end. Rapid tube failure can result.
U.S. Pat. No. 4,588,022 discloses an anodically protected heat exchanger in which a negative potential is provided to both ends of a cathode that extends from one end of the exchanger to the other. In order to control the potential profile on the tubes along the entire length of the exchanger within the passive range, a variable resistor is provided in the electrical connection between the negative terminal of the direct current power source and the cathode at the cold end of the exchanger. Since the hot end typically draws more current, the voltage drop in the resistor allows the potential at the negative terminal of the power source to be controlled at a level sufficient to establish a passive voltage on the exterior surfaces of the tubes in the hot end without straying into the transpassive range in the cold end.
According to U.S. Pat. No. 4,588,022, the cathode may also be encased in a Teflon sheath which prevents grounding of the cathode on the metal parts of the exchanger and avoids transpassivity on the baffles and tube sheet in close proximity to the cathode. Holes in the Teflon sheet allow for passage of current between the metal parts to be protected and the portions of the cathode rod sufficiently distant from tube, tube sheet, and baffle surfaces to avoid grounding or transpassivity.
In an alternative embodiment, the '022 patent describes a system in which an independent power source is attached to each end of the cathode, each power source having an independent controller.
Although the anodic protection system of the '022 patent provides advantages over systems which utilize a multiplicity of pin cathodes, control of the voltage profile along the length of the heat exchanger is unavoidably limited by exposure of a single cathode to the corrosive liquid system along the entire length of the exchanger. Where the exchanger is of substantial length and/or the temperature differential from end of the exchanger to the other is very large, the operation of the variable resistor may not be effective to control the voltage profile so that it nowhere strays outside the passive range. With colder acid conditions wherein current requirements and voltage drop through the resistor falls toward zero, and the resistor and the resistivity of the cathode lose their regulatory effect. Even where independent power sources are used, it may not always be feasible to control both ends of a single cathode at voltages which preserve the voltage within the passive range along the entire length of the cathode. Under low current conditions, the resistivity of the cathode is inadequate to prevent the voltage applied at the hot end of the cathode from prevailing along the entire length of the cathode.
In an effort to facilitate independent control of voltage at the respective ends of the exchanger, the '022 patent uses a cathode having a defined range of resistivity. The patent expressly avoids the use of copper core cathodes, for example, so that the cold end of the exchanger is not shunted to the same potential as the hot end. However, this very resistivity necessarily creates a significant voltage gradient, which in a long exchanger may cause the null point (i.e., the point of minimum voltage) to remain in the active range if the voltage at that point of the exchanger is inadequate Lo form the passive film.